In many chemical processes it is necessary to react slurries at high pressure. Examples are pressure oxidation of ores and polymerization of monomers, for example, ethylene, in the presence of solid catalysts. There are particularly many such reactions in the field of coal conversion, that is, gasification, liquefaction, and more recently, combustion. Typically in such reactions, it is necessary to transfer the solids from a high-pressure zone to a low-pressure zone in order to process them further or discard them. If the pressure reduction occurs across a valve, extremely rapid erosion may occur, particularly when the valve is first "cracked," open and solid particles move at very high velocities. Valves in this service have been known to fail in a matter of minutes.
It is widely recognized in coal conversion research that slurry pressure letdown creates a severe erosion problem for valves. A number of research programs have been directed toward solving the problem either by new valve designs or development of superior materials of construction. Entire symposiums have been devoted to corrosion/erosion problems in coal conversion. Typical papers are "Corrosion/Erosion of Materials in Coal Liquefaction Environments" by Alan Levy and "Erosion Resistance of Ceramic Materials in Coal Slurry Letdown Service" by John J. Mueller, both included in the proceedings of Corrosion/Erosion of Coal Conversion Systems Materials Conference, Berkeley, Calif. Jan. 24-26, 1979. The petroleum production industry has faced a similar problem in brining up oil from reservoirs at great pressures and discharging it to storage at atmospheric pressure. Entrained sand particles create an erosive slurry. A key component developed by the industry has been the choke, a small diameter tubular restriction placed in the letdown line to take the major pressure drop. Descriptions of the design and use of chokes are well presented in "Petroleum Production," Wilbert F. Cloud, University of Oklahoma Press, 1939.
Means of circumventing the valve erosion problem in high-pressure slurry systems have been developed, in particular, "lock hoppering" wherein a slurry receiving vessel, or lock hopper, is equalized in pressure with the reactor. The slurry is then discharged batchwise to the receiving vessel by gravity flow. The receiving vessel is then valved off from the reactor and depressurized by venting the gases or vapors above the slurry level. A bottom discharge valve on the receiving vessel may then be opened and the slurry discharged by gravity. In practice, two receiving vessels are used so that one is collecting slurry while the other is discharging. This approach, however, has disadvantages of its own in that depressurizing may result in the loss of valuable product vapors or require special handling of hazardous gases. The valves must be operated frequently and incur rapid wear, particularly at high pressure when cycles are short. This type of device does not operate in a truly continuous manner. In spite of development of novel valve designs and more durable materials of construction, the excessive erosion of pressure letdown valves has remained a serious deterrent to the development of high-pressure coal conversion systems.